Cellular materials find much use in applications in which structural materials having both strength and light weight are required. Generally, a sheet of cellular material is formed from a plurality of thin strips of ribbon formed of one of many structural materials such as aluminum, stainless steel, paper, etc. Typically, each strip is formed into a periodic corrugated shape and the component strips are stacked and fastened together at abutting surfaces to form a matrix comprising a repeating pattern of cell units, often substantially identical cell units, which define the sheet. Typically, cellular material is made up of ribbon defining cell units having a honeycomb (hexagonal), or square shape, or derivatives thereof.
Unless fabricated specifically to conform to a particular predetermined shape, such a sheet is typically substantially flat, and reference may be made to the sheet according to x and y axes defining a plane parallel to the sheet, and a z-axis which passes through the sheet perpendicularly. Typically, the strips of ribbon which make up cellular material are in a substantially perpendicular relation to the x-y plane, and one advantageous characteristic of such cellular material is that it has a high strength-to-weight ratio along its z-axis.
Fabrication of cellular material may be carried out in a number of ways and typically involves ribbon corrugation and subsequent assembly of component strips. U.S. Pat. No. 4,632,862, incorporated herein by reference, discloses two common methods of corrugation: rolling and stamping. According to the rolling method, strips of ribbon are fed through cooperating forming rolls having complementary peripheral teeth which mesh together upon the ribbon to form the desired corrugations. According to the stamping method, ribbon is corrugated as it is fed through a die stamping machine. At the completion of the stamping or rolling process, component strips of ribbon material are stacked such that the lowest sections of each corrugated strip contact the highest sections of the corrugated strip upon which it is stacked, and the contacting sections, or nodal junctions, are fastened by means such as brazing, welding, or adhesion. According to another method of fabrication disclosed in U.S. Pat. No. 3,086,624, incorporated herein by reference, one continuous ribbon is fan-folded at regular intervals to form a sheet of cellular material.
According to each of these methods, the thickness of a sheet of cellular material (that is, its dimension along the z-axis) is defined by the width of the component strips of ribbon from which it is fabricated, and the length and width of the sheet, its dimensions along the x and y axes, are defined by the length of the ribbons after corrugation formation (or the length of each fan-folded segment according to that method), and the number of strips incorporated multiplied by the overall depth of corrugation of each strip of ribbon (or each fan-folded segment).
Another well-known method of fabrication of cellular material involves stacking multiple flat strips of ribbon in a manner such that each strip is fastened at a first set of regular intervals along its length to the strip upon which it is stacked, and fastened at a second set of regular intervals along its length to the strip which is stacked upon it, the second set of regular intervals falling midway between the first set of regular intervals. The material is then mechanically expanded by pulling the first strip and the final strip of the stack apart. According to the method, the unjoined strip sections separate to define open cell units having a hexagonal, roughly hexagonal, or roughly square shape.
Examples of applications for which cellular materials are ideally suited include structures in aircraft and ships; in the construction industry for contoured and flat laminate structures; for surrounding high-pressure containment structures for safety reasons, for example steam pipes in nuclear power plants; for electromagnetic insulation, for example to insulate sensitive instrumentation from radio waves; for flow control applications; for sound insulation; and as structures to increase turbulence in labyrinth seals.
In many of these and other applications, it is desirable to employ a cellular material which may be routinely fabricated as a substantially flat sheet, but which is easily conformable to cylindrical or other curved surfaces (such as jet aircraft engines for sound insulation or steam pipes in nuclear facilities), or to highly irregular surfaces. However, heretofore available cellular materials are not easily formable to such surfaces, but commonly must be fabricated to conform specifically to a particular surface shape.
The lack of formability of known cellular materials is due to their inherent anticlastic behavior. Such behavior is defined by a material's tendency to "saddle" in a direction perpendicular to the direction in which the material is bent. That is, when the material is bent along its x-axis so as to form an arc of a circle having a center on a first side of the material, the material spontaneously curves along its y-axis in a direction so as to form an arc of a circle having a center on the side of the material opposite the first side. Such behavior is most pronounced in cellular material that is relatively stiff along its x and y axes, and may be advantageous in circumstances in which such stiffness is desired. However, this characteristic is highly undesirable in the above-mentioned applications in which material highly conformable to cylindrical or spherical surfaces or highly irregular surfaces, while maintaining a substantially perpendicular relation between the material's ribbon strips and the surface to which the martial is to conform, is advantageously utilized. U.S. Pat. No. 3,340,023, incorporated herein by reference, discloses a cellular material which is somewhat formable to curved surfaces. However, the material exhibits anticlastic behavior to some extent and is not suitable for many applications.
Accordingly, general purposes of the present invention are to provide a cellular material which exhibits highly synclastic behavior, that is, which is highly formable without saddling, to provide a cellular material which may be easily and inexpensively fabricated as a substantially flat sheet and which may then conform to any of a variety of surface shapes; and to provide a method of fabrication of such material.